Proximity-detectors for parking meters and the like



May 11, 1965 'r. L. PALFI 3,

PROXIMITY-DETECTORS FOR PARKING METERS AND THE LIKE Filed Dec. 27. 1961 in E SWITCH v 1 4 I AND 22 2d 158- GATE 26 6 PARKING FLIP-FLOP \m I 2 :METER (BISTABLE) s e zg- 4- 16a 4 0 w 26 40 (L JWEZA :33 m E 1 Th1 g FIG 4A INVENTOR.

THOMAS L. PALFl floL 5.724 29;

ATTORNEY 'nism of the parking meter. the invention resides in the provision of a proximity detec- United States Patent "ice 3,183,411 PRGXIMITY-DETECTORS FOR PARKING METERS AND THE LIKE Thomas L. Palfr, Brockton, Mass, assignor to Corneil- Dubilier Electric Corporation, a corporation of Delaware Filed Dec. 27, 1961, Ser. No. 163,011 Claims. (Cl. 317-123) The present invention relates to proximity detectors and provides apparatus which is particularly well suited to effect automatic reset of unexpired time registered in a parking meter. While certain aspects of the invention have broad application they will be disclosed below in connection with their application to parking meters.

However, except where the context so requires, this is not to be construed as limiting.

Parking meters are commonly installed at spaced points corresponding to vehicle parking spaces. 'It is customary for a motorist to park his car next to a meter, and the motorist then deposits a coin in the meter to start an authorized-parking time interval.

Motorists often leave a metered parking space before the allowed time has expired; and then it is possible for another motorist to park in'the same space, without depositing another coin. This is contrary to most parking regulations and yet, because of lack of etfective enforcement techniques, the practice is condoned.

A feature of the present invention is the provision of new and effective automatic reset apparatus for the timing means of parking meters. More particular an object of this invention resides in the provision of novel vehiclesensing apparatus for controlling automatic reset mecha- An object of this aspect of tor having directional characteristics. It will be appreciated that, where multiple parking meters are located to serve adjacent parking spaces, each of such meters should be equipped with selectively effective proximity detectors. In that Way, each meter, when equipped with an automatic reset control, will respond only to the corresponding proximity detector.

A further object of the invention resides in the provision of a proximity detector that is compensated for reliable operation over a wide range of environmental and other changes.

Automatic reset control for parking meters has been recognized as highly desirable. However, a defect might develop which would interfere with proper operation of the reset apparatus. Such a defect could upset the normal operation of the parking meter itself. A motorist might park adjacent a parking meter and then deposit the required coin for starting an allocated timing cycle; but because of a hypothetical flaw in the automatic reset control apparatus, the parking meter might immediately be reset to Zero.

1 To avoid such a condition, it has been suggested that automatic reset control apparatus for parking meters should have fail-safe characteristics. An object of the present invention resides in the provision of a novel and highly eifective fail-safe system for automatic reset control apparatus for parking meters.

A further related object resides in novel proximity detection apparatus having a compensated proximity detector. Another related object resides in the provision of proximity sensitive control apparatus having novel fail safe control means for disabling spurious indication of the condition to be detected when such spurious indication results form any of a wide variety of defects in the detection apparatus itself.

The foregoing objects and features of the invention are achieved in the illustrative embodiment that is described 3,183,411 Patented May 11, 1965 in detail below and shown in the accompanying drawings. This illustrative embodiment includes a four-arm capacitance bridge having a sensing capacitor that is exposed in a sensing region and is thus subject to changes in its capacitance resulting from ambient temperature changes. The bridge includes another capacitor that is shielded and is thus non-proximity-sensing but is proportionally responsive to the same changing ambient temperatures and to conditions producing drift so as to compensate tor such changes in the sensing capacitor.

The sensing bridge is excited by a high frequency source, and is balanced in one condition (when the parking space is not vacant) and is unbalanced in another condition (when the parking space is occupied). The proximity detection apparatus could conceivably develop a variety of defects that would produce spurious output corresponding to the former condition (parking space not occupied) even though the latter condition prevails (parking space occupied). The apparatus described below includes a dual output control having one input control connection evidencing the proper working condition of the proximity detection apparatus at the start of a detection interval, and the dual output control has another input control connection that is responsive to the change of the proximity detector from the starting condition to the opposite condition. Thus the detection apparatus has the fail-safe characteristic of providing assurance that the detection apparatus is in proper working order when the vehicle is parked so that, when that vehicle departs, the changed response of the proximity detector is meaningful. The entire detection apparatus is disabled if, at the start of the detection interval, the apparatus is not in proper working order. Further, even if the detection apparatus is in proper condition to satisfy the star-ting requirements of the dual output control, there will be no automatic reset unless the detection apparatus is operative to revert properly to that condition representing departure of the parked vehicle.

The nature .of the invention and its various objects and features will be more fully appreciated from the detailed description of the illustrative embodiment which appears 1 elow and which is shown in the accompanying drawings forming part of this disclosure. In the drawings:

FIG. 1 is a perspective view of a typical parking-meter installation modified to incorporate an embodiment of the present invention;

FIG. 2 is an enlarged horizontal cross section through the apparatus of FIG. 1 at the-plane 22;

FIG. 3 is a block-diagram of a control circuit embodying features of the present invention;

FIG. 4 is a wiring diagram of a presently preferred control circuit embodying various features of the invention, generally corresponding to the block diagram in FIG. 3; and

FIG. 4A is a modified portion of the wiring diagram ofP IG. 4.

Referring now to the drawings, particularly to FIG. 3, a proximity detector 10 is shown excited by a high frequency oscillator 12. Proximity detector 10 is in the form of a balanced impedance bridge, yielding no appreciable output under normal conditions but adapted to the unbalanced by proximity of a vehicle in the sensitive space near thesensing bridge.

An amplifier 14 and rectifying amplifier 18 supply bridge output in one channel via switch means 50 to an and gate 16, and in another channel to a bistable device or flip-flop 20 whose output is connected to another control point of the and gate 16. A reset control point for flip-flop 20 is connected to a parking meter 22 for control thereby.

As seen in FIGS. 1 and 2, a sensing portion Ida of the bridge is mounted on a pipe 24 supporting the parking meter. Pipe 24 is set deep into the pavement and is effectively grounded. At its upper end pipe 24 carries a parking meter 22. Most of the control apparatus of FIG. 2 is advantageously contained within the hollow space of pipe 24. A reset electromagnet 26 is included in the parking meter, effective when energized to restore, the elaspesd time mechanism of the parking meter to zero. The relationship between the reset electromagnet and the interval mechanism of the parking meter forms no part of the present invention. Such arrangements are well known.

The automatic reset system in the block diagram of FIG. 3 operates the reset apparatusof the parking meter, but the system has an important fail safe feature that suppresses its own operation when defective due to most conceivable causes.

In its normal condition, the parking meter is at zero, and power to the reset-control circuit is switched off. Flip-flop 2% is in one of its two stable conditions, disabling and gate 16 from operating the reset electromagnet.

When a vehicle is parked in front of the sensing portion a of bridge 10, a coin is deposited in the parking meter 22 to start a timing interval, setting the elapsed time mechanism at some point away from zero. Concurrently, a switch is closed for supplying power to the automatic reset control apparatus, normally starting oscillator 12 in continuous operation. When sensing bridge 10 is unbalanced by a parked vehicle, and the control circuit is energized, a signal is developed in amplifier 14 that is applied both to and gate 16 via switching device 50 and to flip-flop 20. This reverses the condition of flip-flop 2d, and puts one connection 16b of and gate 16 in condition to operate reset electromagnet 26. The control effect of amplifiers 14 and 18 is transmitted to the other control connection 16:: of and gate 16, placing that control connection in its reset-suppressing condition.

So long as the vehicle remains parked, sensing bridge 10 remains unbalanced and the disabling signal applied to control connection 16a of the and gate suppresses operation of the reset electromagnet 26. When the vehicle leaves the region of the sensing bridge 11), the output of amplifier 14 ceases. Amplifier 18 resumes its initial state, but this does not disturb the switched state of flipflop 20. Connections 16a and 16b are thus in their proper states for and gate 16 to function, so that reset electromagnet 26 is operated. As a result, the timing mechanism of the parking meter is reset to zero.

It appears from the foregoing that, so long as the bridge remains unbalanced, there is a reset-suppressing signal on connection 1612. When the vehicle departs and the bridge is once again balanced, the reset means is enabled to function. With such operation it would appear that flip-flop 28 as well as gate 16 could be omitted. However, these parts provide an important fail safe function. To demonstrate their importance, let it be assumed that flip-flop 2t) and its and gate connection 161) were omitted.

There is a possibility of oscillator 12 being inoperative, or of bridge 11) being nearly balanced when a vehicle is present, and there is the possibility that, for some other reason, the signal at connection 16a is inadequate to prevent the reset gate from functioning even though a parked car remains in proper proximity to the sensing bridge. Thus, if any or all of the control apparatus including elements 10, 12, 14, 18 and 16 were inoperative, a car might be parked, a coin might be inserted to start a timing interval and thereupon the meter would be reset immediately. This is most objectionable, and this result is avoided by unit 20 and and gate 16. Because of these latter elements, reset cannot occur unless the circuit 19-12-14 is initially operative to trigger flip-flop 21] into its resetenabling condition. Thereafter, when the signal from channel 10-12-14 to gate connection 16a disappears, reset occurs properly. With the block diagram shown, the

entire reset apparatus that is controlled by the proximity sensing device 10 is disabled from interfering with normal operation of the parking meter if the reset apparatus is not in proper working order at the start of the timing interval. 7

Referring now to FIG. 4 there are illustrated the practical details of an illustrative embodiment of the circuit represented in the block diagram of FIG. 3.

High-frequency oscillator 12 (for example, 465 kc.) is coupled by its tuned interstage coupling transformer 12a, appropriately shielded, to a four-arm impedance bridge 11 The bridge includes a sensing arm 10a, a shielded compensating arm 10b, and two balancing arms 10c and 10d. Resistors 28 between transformer 12a and the input points 31) and 32 of the bridge prevent the capacitance bridge from excessively loading oscillator 12. So long as power is applied, the oscillator signal is continuously applied to bridge input terminals 3G and 32. Bridge output points 34 and 36 are coupled by a shielded and tuned interstage transformer 14a to amplifier stage 14 whose output interstage transformer 14b is coupled to a self-rectifying transistor amplifier 18.

Amplifier 18 utilizes a PNP transistor with groundedemitter connections. Thus, base 18a is connected to one terminal of the secondary winding of transformer 14b, the other secondary terminal being grounded at the positive terminal of the direct-current supply. Emitter 18c is similarly grounded. Collector 18b is connected to moving contact arm 40 of a relay that includes two coils 42 and 44. So long as the relay is not energized, moving contact 40 engages contact 40a that is connected to one terminal of relay winding 42. The opposite terminal of this winding extends to the negative terminal of the direct current supply.

In the absence of signal, there is no significant output from amplifier 18, and when signal is applied to its base, amplifier 13 develops a substantial current of rectified pulses.

Moving contact 40 is operable to engage another con tact 401) when the relay is energized. This connects collector 18b to its load resistor 46 and, through this resistor, to the negative terminal of the direct current supply.

Another moving contact arm 48 of the relay is connected to the negative terminal of the direct current 'supply; and contact :8 is movable when the relay is energized to engage contact 48a that is connected to one terminal of the relay holding coil or Winding 44. The other terminal of winding 44 is grounded at the positive supply terminal. When either coil of the relay is energized, contact 48 engages contact 4301, to provide a holding circuit for the relay.

Transistor 50, type PNP, has its base connected to the terminal of resistor 46 that is connected to contact 4% and, when the relay is energized, to collector 18b of transistor 18. The emitter of transistor 50 is connected to the positive supply-terminal, and the collector of transistor 50 extends to one terminal of reset electromagnet 26. The other terminal ofreset electromagnet 26 extends to contact 48a of the relay. As will appear, the various components at the lower portion of the circuit in FIG. 4 carry out the logic functions represented by portions 16, 20 and 5% of the block diagram in FIG. 3.

Switch 22a, controlled by the parking meter, is interposed between the negative terminal of the direct-current supply and the negative line of the above circuit. Contacts 22a are diagrammaticall illustrated as being open when the elapsed time indicator 22b is in its zero position. Whenever the indicator 22b is out of the zero position, contacts 22a close.

In the at-rest condition of the apparatus, with contacts 22a open, the relay is tie-energized. Accordingly, contacts 40 and 48 are in the positions illustrated. Relay windings 4-2 and 44, and relay contacts 40, 40b, 43 and 48a may be regarded as a bistable device, which is in one condition in the configuration illustrated. Contacts 4848a are arranged as a gating connection forreset electromagnet 26, this gating connection being open in the at-rest condition of the apparatus. Another gating connection to electromagnet 26 is established by amplifier 50, which is connected to amplifier 13 when contacts 40 and iilb are closed. Transistor 18 is connected so that, even with switch 22a closed, no appreciable collector current is drawn. Conversely, transistor 5% is connected so that it is switched on when power switch 220 is closed and with no signal being supplied to transistor 18. The emitter-collector circuit is, however, open at contacts 48, 48a before switch 22acloses; and after switch 22a closes, assuming the proximity detection apparatus is operative, the switched-on condition of transistor 18 draws enough current through resistor 46 to switch transistor 50 off. Capacitor 18d smooths the half-wave output pulses of transistor 18 when there is signal input to this transistor.

A typical sequence in the operation of the circuit in FIG. 4 may now be reviewed. At the outset, switch 22a is open since the parking meter is assumed to be at its zero indication. After a vehicle is parked in proximity to sensing capacitor a, the bridge is unbalanced, and a coin is inserted in the meter. The indicator 22b moves away from zero, switch 22a closes, and the circuit of FIG. 4 is energized.

Because the bridge is unbalanced by a parked car, a signal from oscillator 12 is transmitted via amplifier 14 to base 18a of transistor 18. The nearly-zero current of collector 18b rises to attain a substantial average value. As a result, current flows via contacts 40, 40a to control winding 42 of the relay. Contacts 4% and 43 then move to engage contacts 40b and 48a, respectively, and a relay holding circuit is established through contacts 4% and 48a and holding coil .-4 of the relay. Closing of contacts 48, 48a also provides a return circuit for reset electromagnet 26 to the negative supply terminal. However, at the same time, the base of transistor 50 is abruptly shifted toward the potential of the positive terminal. This results from the suddenly inceased drop in resistor 46 due .to current drawn by collector 18b via contacts 49, 40b.

Transistor 5%) which previously was switched on by the connection of its base to a strongly negative'control potential-point is suddenly switched off. The conductivity of transistor 50 remains at a low average value so long as output signal from amplifier 14 continues to drive biased-off amplifier 18 into substantial average conduction.

Switching of the bistable relay circuit in such manner as to open contacts 4%), 4th: and de-energize coil 42 establishes the relay holding circuit and renders the bistable device no longer responsive to the signal delivered by the sensing-bridge circuit.

If the vehicle should leave the region of proximitysensin capacitor 10a before the time of the parking meter has elapsed, then the signal to transistor 18 disappears and its current is substantially cut off while the collector current of transistor 59 is switched to a high average value. An energizing circuit for reset electromagnet 7.6 may then be traced from the positive supply terminal via the emitter and collector of transistor 56, through electromagnet 26 to closed contacts .811, 48, and to the negative supply line. Operation of electromagnet 26 resets the parking meter to zero.

. Reset of the parking meter causes contacts 22a to open. When this occurs, current to holding coil 44 of the relay is interrupted, and the moving contacts 4%) and 48 resume thepositions illustrated in FIG. 4.

Comparing the logic aspects .of F IG. 4 with that of FIG. 3, it is seen that transistor 18 has a current drain which is either negligibly low in the absence of bridge unbalance or high when the bridge is unbalanced. Amplifier 38 has .a control connection via contacts 40, 40a to winding 42 of the relay, and is thus effective to reverse the bistable relay into its stable gate-operating condition when the bridge is unbalanced by a parked car. Before the relay was energized, its open contacts 48, 48a perform the gating function of preventing energizing current from reaching connection 15!; of coil 26. After the relay is initially energized, contacts 48, 43a are closed and one side of electromagnet as is energized. The other connection 16a of that electroniagnet is, however, switched ofr' inasmuch as the signal to transistor 18 causes heavy collector current to be drawn through resistor 46 and thus switching transistor 50 oil.

If the parked vehicle should depart before the time interval registered in the parking meter returns the indicator to Zero, the signal to transistor 18 would disappear because the capacitor bridge would become rebalanced and, consequently, amplifier 50 would resume its highconductivity condition. Transistor 50 would thus switch the other terminal lead lea of electromagnet 26 into lowesistance connection to the positive direct-current supply terminal while the previously established circuit from the other terminal 161) of electromagnet 26 extends through closed contacts 43, 48a to the negative supply terminal. Both gated connections for electromagnet 26 being satisfied, this electromagnet is energized.

Only if the entire detection circuit is operating at a level sufficient to reverse the condition of the bistable relay at the start of the timing interval will the automatic reset apparatus be called into operation. Further, if the sensing bridge should remain unbalanced, for some unexplained reason, despite departure of the vehicle from the sensing region of the parking meter, no automatic reset operation would occur. Consequently, the apparatus described provides a high degree of fail-safe characteristics. This-provides assurance that the individual who deposits the necessary coin for starting a timing interval of the parking meter will not be deprived of the allotted parking time because of numerous remotely conceivable causes of malfunction or even complete failure of the automatic-reset control apparatusdescribed.

Transistor 58 is switched off and on, in dependence upon the opposite conditions of transistor 13, and accordingly transistor 5%) may be regarded as an inverter amplifier, or as a switching device. This aspect of its performance is demonstrated by the modification in FIG. 4A, wherein transistor Si? is replaced by relay 50' having a control connection Etla that corresponds to the base of transistor 50, and contacts 5% which correspond to the emitter-collector circuit of transistor 51' Transistor 5%) may be replaced by the relay of PEG. 4A. Correspondingly other substitutions may be made in the circuit of FIG. 4 for carrying out the functions described and, correspondingly, for implementing the block diagram in FIG. 3.

Despite the provision or" the fail-safe features of the automatic reset control apparatus, the primary function or this apparatus is to perform in the intended manner for automatically resetting the timing mechanism of the parking meter to zero when the parked vehicle departs. One possible cause of malfunction would be the disabling of the sensing bridge as a result of aging or exposure of the sensing capacitor to environmental conditions that would change its capacitance and thus unbalance the bridge when the parking space is vacant. To avoid this the bridge in FIG. 4 is advantageously of a physical form that incorporates both the sensing capacitor 10:: and a reference or compensating capacitor 1% in a single unit and formed of the same materials so as to be subjected to the same ambient conditions that -ight produce bridge balance. Arms Eda and 1% of the bridge are physically constituted in the manner illustrated in FIGS. 1 and 2. Three metal foils 54, 5d and 53 are separated from each other by layers as and 62 of the same dielectric or insulation. Foil 64 is disposed midway between foils 56 and 58, within dielectric layer 62. The vertical edges of foils 56 and 53 are joined by vertical connections 65 and 68, and the top and bottom edges of foils 56 and 58 are similarly connected. As a result, electrode 64 is spaced from interconnected electrodes 56 and 58 and forms a shielded capacitor lttl'b, having terminal connection 3 to foil 64. Foils 54 and 5d and the dielectric layer 6% constitute sensing capacitor ma. Connection 32 is formed tofoil '54. Pipe 24 contacts foil 53 and is connected to foil 56 so as to constitute ground terminal 34. The entire unit on pipe 24, consisting of layers of foil and dielectric, are advantageously formed as a rigid unit, the dielectric being cast or molded of a plastic that bonds to the foil layers. The thickness of the layers comprising capacitors ltia and ltib is exaggerated in FIG. 2. In practice the entire unit is thin enough so that no significant temperature gradient would develop in normal use.

In an example, the structure mounted on pipe 24 occupies a 120 arc and provides directional sensitivity for the sensing capacitor ltta. When a vehicle is parked in the vicinity of the parking meter, the vehicle increases the capacitance between foil 54 and ground, as represented by capacitance Etta; with resulting unbalance in the bridge.

Because the same dielectric material is used at layers 6% and 62, the sensing capacitor 1% will vary in capacitance in the same manner as reference or compensating capacitor b. Such variations may result from aging of the dielectric, or from changes in ambient temperature. The material of capacitors lite and ltid should correspondingly be of likematerials, to avoid disproportionate effects due to temperature and aging.

The foregoing illustrative disclosure represents the presently preferred embodiment of the invention in its various aspects. However, it will be appreciated that this disclosure is susceptible of numerous changes in detail and of varied application of its several novel features. Consequently, the invention should be broadly construed in accordance with full spirit and scope.

What I claim is:

1. A control circuit for a parking meter having a reset electromagnet, including vehicle-responsive means adapted to be shifted from its normal condition in the absence of a vehicle to a reset-suppressing condition when a vehicle is in control proximity thereto, bistable means, said vehicle responsive means and said bistable means being connected to said electromagnet in and control relation thereto, and a control connection from said vehicle responsive means to said bistable means effective to set said bistable means in its stable electromagnetoperating condition when said vehicle-responsive means is properly operative and in said reset-suppressing condition, whereby when said vehicle responsive means responds properly to the departure of a vehicle and consequently shifts to its normal condition both the bistable means and said vehicle responsive means are in condition to cause operation of the electromagnet.

2. A control circuit for a reset electromagnet in a parking meter, including vehicle-responsive means adapted to be shifted from its normal condition in the absence of a vehicle to a reset-suppressing condition when a vehicle .is in control proximity thereto, bistable means adapted to act jointly with said vehicle responsive means for controlling said electromagnet, and a control connection from said vehicle-responsive means to said bistable means eflfective to set said bistable means in its stable electromagnet-operating condition when said vehicleresponsive means is properly operative and in said resetsuppressing condition, whereby when said vehicle responsive means again shifts to its normal condition upon departure of the vehicle, both the bistable means and said vehicle responsive means are in condition to cause operation of the electromagnet.

3. A control circuit including an electromagnet, proximity-detecting means adapted to be in one condition or a 8 another condition in dependence on the proximity of a body to said detecting means, bistable control means, both said bistable means and said detecting means being connected in .and control relation to said electromagnet, and a control connection from said detecting means to said bistable means effective to set said bistable means in its stable electromagnetic:operating condition when said detecting means is properly operative and in said electromagnet-supp-ressing condition, whereby when said detecting means shifts to its other condition, both the bistable means and said detecting means are in condition jointly to cause operation of the electromagnet.

4. A control circuit for an output device, including a proximity-detector adapted to be shifted from one condition to another condition in dependence upon the presence or absence of a body to be detected, bistable means adapted to act jointly with said proximity detector for controlling said output device, and a control connection from said proximity detector to an input control portion of said bistable means effective to set said bistable means in its stable output-device-operating condition when said proximity detector is properly operative and in its output-device-suppressing condition, whereby when said proximity detector is shifted into its other condition, both the bistable means and said proximity detector are in condition to cause operation of said output device.

5. A control circuit for a parking meter having a reset electromagnet, including an impedance bridge including a vehicle-responsive sensing capacitor adapted to be shifted from its normal balanced condition in the absence of a vehicle to an unbalanced condition When a vehicle is in control proximity thereto, exciting means for said bridge, unbalance detecting output means for said bridge, bistable means, said unbalance detecting out put means and said bistable means being connected to said electromagnet in and control relation thereto, and a control connection from said unbalance detecting output means to said bistable means effective to set said bistable means in its stable'electromagnet-operating condition when said exciting means and said bridge and said output means are properly operative and in said unbalanced condition, whereby when said bridge responds properly to the departure of a vehicle and consequently shifts to its normal balanced condition both the bistable means and said bridge output means are in condition to cause operation of the electromagnet.

6. A control circuit for a reset electromagnet in a parking meter, including an impedance bridge having a vehicle-responsive capacitor, said bridge being adapted to be shifted from its normal balanced condition in the absence of a vehicle to an unbalanced reset-suppressing condition when a vehicle is in control proximity thereto, bridge exciting means and unbalance detecting means connected to said bridge, bistable means adapted to act jointly with said bridge output means for controlling said electromagnet, and a control connection from said bridge output means to said bistable means effective to set said bistable means in its stable electromagnet-operating condition when said bridge-exciting means and said bridge and said bridge output means are properly operative and in said reset-suppressing condition, whereby when said bridge shifts to it normal balanced condition upon departure of the vehicle, both the bistable means and said vehicle responsive means are in condition to cause operation of the electromagnet, and means coupled to said bistable means and operative upon reset actuation of the parking meter for setting said bistable means in its reset-suppressing condition.

7. A control circuit for an output device, including a proximity-detector adapted to be shifted from one condition to another condition in dependence upon the presence or absence of a body to be detected, bistable means adapted to act jointly with said proximity detector for controlling said output device, a control connection from said proximity detector to an input control portion of said bistable means effective to set said bistable means in its stable output-device-operating condition when said proximity detector is properly operative and in its output-device-suppressing condition, whereby when said proximity detector is shifted into its other condition, both the bistable means and said proximity detector are in condition to cause operation of said output device, and further means connected to said bistable means and operative from time to time for setting said bistable device in its output-device-disabling condition.

8. A control circuit including an electromagnet, proximity detecting means including a bridge having tWo c-apacitive elements one of which is shielded and the other of which is exposed and thus proximity-sensitive, said bridge being balanced or unbalanced in dependence on the proximity of a body to said exposed capacitive element, bistable control means, both said bistable means and said detecting means being connected in and con trol relation to said electromagnetic, and a control connection from said detecting means to said bistable means effective to set said bistable means in its stable electromagnet-operating condition when said detecting means is properly operative-and in its electromagnet-suppressing condition, whereby when said detecting means shifts to its electromagnet-operating condition, both the bistable means and said detecting means are in condition jointly to cause operation of the electromagnet, and further periodically operable means connected to said bistable means in control thereof to set said bistable mean in its electromagnet-disabling condition.

9. A control circuit including an electromagnet, proximity detecting means including a bridge having two capacitive elements one of which is shielded and the other of which is exposed and thus proximity-sensitive,

said elements being of like materials so as to respond alike to aging and to temperature changes, and said elements being disposed in substantially the same environment, said bridge being balanced or unbalanced in dependence on the proximity of a body to said exposed capacitive element, and a coupling from said proximity detection means to said electromagnet.

10. A control circuit for a parking meter having a reset electromagnet, including an impedance bridge having an unshielded vehicle-responsive capacitive element and a shielded reference capacitive element, said elements being of like materials so as to age alike and to respond alike to ambient temperature changes, and said elements being disposed substantially at the same location for exposure to substantially the same environment, said bridge being adapted to be shifted from its normal balanced condition in the absence of a vehicle to an unbalanced condition when a vehicle is in control proxrrnity thereto, exciting means for said bridge, unbalance detecting means for said bridge, and coupling means between said unbalance detecting means and said reset electromagnct.

Reierences (listed by the Examiner UNITED STATES PATENTS 2,652,551 9/53 Gumperz et al 340-51 2,994,019 7/61 Franz et al. 317-246 3,015,050 12/61 Deyerl 317-242 3,050,662 8/62 Miller et al. 3l7148.5 3,080,509 3/63 Rowe 317-149 3,125,751 3/64 Winters 340258 SAMUEL BERNSTEIN, Primary Examiner. 

1. A CONTROL CIRCUIT FOR A PARKING METER HAVING A RESET ELECTROMAGNET, INCLUDING VEHICLE-RESPONSIVE MEANS ADAPTED TO BE SHIFTED FROM ITS NORMAL CONDITION IN THE ABSENCE OF A VEHICLE TO A RESET-SUPPRESSING CONDITION WHEN A VEHICLE IS IN CONTROL PROXIMITY THERETO, BISTABLE MEANS, SAID VEHICLE RESPONSIVE MEANS AND SAID BISTABLE MEANS BEING CONNECTED TO SAID ELECTROMAGNET IN "AND" CONTROL RELATION THERETO, AND A CONTROL CONNECTION FROM SAID VEHICLE RESPONSIVE MEANS TO SAID BISTABLE MEANS EFFECTIVE TO SET SAID BISTABLE MEANS IN ITS STABLE ELECTROMAGNETOPERATING CONDITION WHEN SAID VEHICLE-RESPONSIVE MEANS ITS PROPERLY OPERATIVE AND IN SAID RESET-SUPPRESSING CONDITION, WHEREBY WHEN SAID VEHICLE RESPONSIVE MEANS RESPONDS PROPERLY TO THE DEPARTURE OF A VEHICLE AND CONSEQUENTLY SHIFTS TO ITS NORMAL CONDITION BOTH THE BISTABLE MEANS AND SAID VEHICLE RESPONSIVE MEANS ARE IN CONDITION TO CAUSE OPERATION OF THE ELECTROMAGNET. 